1. Field of the Invention
This invention relates to fructan (levan) polyfructose polymers and methods for their preparation using strains of the bacteria Bacillus polymyxa (B. polymyxa). Levans are natural polymers of the sugar fructose, broadly (or generally) called fructans, in which the D-fructofuranoside monomeric units are linked by a .beta.,2-6 bond. These polymers are found in many plants and microbial products and are useful as emulsifying and thickening agents in the food industry.
2. Description of the Prior Art
Fructans occur naturally in two general forms distinguished by the type of linkage between the fructose molecules as illustrated: ##STR1##
Inulin, the form found in many plants, is formed with a backbone of .beta.,2-1 linked fructose molecules. Levans, formed as microbial products, have a backbone of .beta.,2-6 linked fructose molecules. Plant fructans (phleins) have shorter residues (about 100 residues) than microbial levans that contain up to 3 million residues [Pontis et al., Biochemistry of Storage Carbohydrates in Green Plants, Dey and Dixon (eds.), Ch. 5, (1985), p. 205, Academic Press, New York]. Microbial levans are produced from sucrose-based substrates by a variety of microorganisms: Acetobacters, Loewenberg, et al., Can. J. Microbiol., Vol. 3, (1957), p. 643; Achromobacter sp., Lindberg, G., Nature, Vol. 180, (1957), p. 1141; Aerobacter aerogenes, Srinivasan, et al., Science, Vol. 127, (1958), p. 143; Phytobacterium vitrosum, Belval, et al., Compt. Rend., Vol. 224, (1947), p. 847 and Vol. 226, (1948), p. 1859; Xanthomonas pruni, Cooper, et al., Biochem. J., Vol. 29, (1935), p. 2267; Bacillus subtilis [Dedonder, R., Meth. Enzymol., Vol. 8, (1966), p. 500 and Tanka, et al., J. Biochem., Vol. 85, (1979), p. 287]; B. polymyxa [Hestrin et al., Biochem. J., Vol. 3, (1943), p. 450]; Aerobacter levanicum (Hestrin, et al., Ibid.); Streptococcus sp. [Corrigen et al., Infect. Immun., Vol. 26, (1979), p. 387]; Pseudomonas [Fuchs, A., Nature, Vol. 178, (1956), p. 921]; Corynebacterium laevaniformans (Dias et al., Antonie Van Leewenhoeck, Vol. 28, (1962), p. 63]. Early reports on levan were obscured by incomplete description of impure products, and yields too low to consider for industrial applications. Levans (fructans) were identified by hydrolysis (acid) yielding .beta.-fructose, and analysis agreeing with the empirical formula (C.sub.6 H.sub.10 O.sub.5).sub.n.
Polysaccharide gums such as the dextrans or xanthan gums are used extensively in the food industry as stabilizers in emulsions and foams such as ice cream, whipped toppings and salad dressings, etc. (Sharma, S.C., J. Food Tech., January 1981, p. 59).
Extracellular polysaccharides produced by microorganisms offer a variety of useful and potentially low-cost industrial gums.
Conventionally, small quantities of levan have been produced by the bacterial fermentation of sucrose using strains of Actinomyces viscosus or Aerobacter levanicum. B. polymyxa generally produce heteropolysaccharides, comprising several different forms of polymers. Genetically engineered Escherichia coli (E. coli) have been produced that will synthesize levan, [Gay, P., et al., J. Bacteriol., Vol. 153, (1983), p. 1424]. Additionally, other methods using aerobic fermentation aimed at the production of levan [Jeanes, et al., U.S. Pat. No. 2,673,828; Gaffor, et al., U.S. Pat. No. 3,879,545; Ayerbe, et al., U.S. Pat. No. 4,399,221] as well as those described above suffer the disadvantages of low yield and contaminating or impure products. As such, there is a need in the industry for an efficient microbial method for increased levan production.